1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system using the same, which are suitable for various types of optical systems, such as an exposure apparatus for use in producing devices, a lighting device, a photographic camera, binoculars, a projector, a telescope, a microscope, a copying machine, a multifunction printer, an image scanner, a facsimile, etc.
In particular, the present invention relates to a diffractive optical element provided in an image reading element, the diffractive optical element being applied to an image reader, such as a copying machine, an image scanner, a multifunction printer, a facsimile, or the like, which comprises a control circuit controlling the relative motion between an original platen and a CCD, and a control circuit controlling the detection signals of the CCD.
2. Description of the Related Art
As one of methods of correcting chromatic aberrations of an optical system, a method has been used wherein two lens materials having dispersion different from each other are combined. In contrast to this chromatic-aberration reducing method by the combination of lens materials, chromatic-aberration reducing methods by using a diffractive optical element having diffractive effect for a lens surface or a portion of an optical system, are disclosed in literatures such as SPIE Vol. 1354, International Lens Design Conference (1990), and in Japanese Patent Laid-Open Nos. 4-213421, 6-324262, U.S. Pat. No. 5,044,706, etc. These disclosures make use of a physical phenomenon that, regarding the way in which chromatic aberrations with respect to a light beam with a reference wavelength occurs in an optical system, there is an inverse relation between a diffractive surface and a refractive surface.
Such a diffractive optical element can provide an effect like an aspherical lens by changing the period of the periodical structure of the diffraction grating thereof, thereby exerting a significant effect in reducing aberrations.
Comparing a diffraction action of a light with a refraction action thereof, one light beam remains one light beam even after refracting on a lens surface, whereas one light beam is split into some light beams with some orders after being diffracted on a diffraction grating.
When using a diffractive optical element as a lens system, therefore, it is necessary to determine a grating structure so that the light flux in an operating wavelength region is concentrated on the light of a particular order (design order). When the light of the particular order is concentrated, the other diffraction lights exhibit low intensities, and when the other diffraction lights exhibit zero intensity, it means that they do not exist. In order for the diffractive optical element to have the above-described features, the diffraction light of a particular order must have a sufficiently high intensity. If there exist lights of diffraction orders other than the particular order, and these lights are made incident on the spots other than those of the light of the particular order, the lights will become flare lights (harmful lights).
In an optical system using a diffractive optical element, therefore, it has been necessary to solve the above-described problem. As solutions to this problem, two types of proposals have been made. One is a laminate-type diffractive optical element having a laminate sectional shape wherein first and second diffraction gratings 103 and 102 are overlaid on a substrate glass 101, as shown in FIG. 6. Another is a laminate-type diffractive optical element, as shown in FIG. 7, wherein first and second diffraction gratings 104 and 105 are formed over substrate glasses 106 and 107, respectively, and wherein the first and second diffraction gratings are overlaid to each other via an air layer so that the pitches of the two mating diffraction grating surfaces correspond between the two surfaces.
The grating thicknesses d1 and d2 of the diffraction gratings 104 and 105 are determined by the refraction indices n01 and n02 of the material, and a reference wavelength xcex0, using the following equation.
(n01xe2x88x921)xc3x97d1xe2x88x92(n02xe2x88x921)xc3x97d2=mxc3x97xcex0xe2x80x83xe2x80x83(1) 
This is an equation used for maximizing the diffraction efficiency of the diffraction light of a m-th order in the reference wavelength xcex0. The refraction indices n01 and n02, and the grating thicknesses d1 and d2 are determined so as to satisfy the above-described relationship.
As producing methods for a diffractive optical element, injection molding, pressure molding, and the like are used.
A method for producing a laminate-type diffractive optical element using injection molding and pressure molding, is disclosed in Japanese Patent laid-Open No. 11-344611.
In a laminate-type diffractive optical element, it is necessary to appropriately set the grating shapes of one diffraction grating and the other diffraction grating. For example, if the distance between the grating surfaces of two diffraction gratings is too wide, the diffraction light of a predetermined order which has been diffracted on one grating surface will not be made incident on the other facing grating surface. On the other hand, if the distance between the two grating surfaces is too narrow, one grating surface will contact the other grating surface during production, thereby imparing the grating surfaces. These make it difficult for the diffractive optical element to achieve a high diffraction efficiency, so that the correcting effect of the diffractive optical element with respect to chromatic aberrations becomes insufficient when used as a portion of an optical system.
Accordingly, it is an object of the present invention to provide a diffractive optical element allowing a high diffraction efficiency to be achieved, and to provide an optical system using the same.
In order to achieve the above-described object, the present invention provides a diffractive optical element formed by laminating a plurality of diffraction gratings, wherein two diffraction gratings adjacent to each other along the incident direction of light satisfy the following relation:
10 xcexcm less than D less than 40 xcexcm, 
when the distance between the uppermost portion of one diffraction grating and the lowermost portion of the other diffraction grating is represented by D (xcexcm).
Preferably, at least one of the above-described two diffraction gratings is molded by injection molding.
Also, it is preferable that the two diffraction gratings be formed by opposingly disposing the surface of one diffraction grating over the surface of the other diffraction grating which has the same pitch as that of the one diffraction grating and which has a grating thickness different from that of the one diffraction grating.
Furthermore, it is preferable that the two diffraction gratings be formed by jointing the surface of the one diffraction grating to the surface of the other diffraction grating face to face via an air layer.
The arrangement may be such that the two diffraction gratings are formed by jointing the surface of the one diffraction grating to the surface of the other diffraction grating face to face via an optical material.
Preferably, the two diffraction gratings are formed of materials of which the dispersion is different between the materials.
Preferably, the diffractive optical element molded by injection molding be formed of a plastic material.
It is preferable that one of the above-described two diffraction gratings be formed of an ultraviolet-cured resin.
Furthermore, it is preferable that one of the two diffraction gratings comprise a substrate and a grating portion formed over the substrate, the grating portion being made of a material different from that of the substrate.
Preferably, the above-described grating portion is formed of an ultraviolet-cured resin.
The above-described substrate may a flat plate, or a curved plate.
Moreover, the present invention provides an image reader comprising a diffractive optical element in accordance with the present invention, and a control circuit which generates the control signals in the image reader.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.